Fine Tuning the Performance of DSSCs by Variation of the π-Spacers in Organic Dyes that Contain a 2,7-Diaminofluorene Donor

Organic dyes that contain a 2,7-diaminofluorene-based donor, a cyanoacrylic-acid acceptor, and various aromatic conjugation segments, which are composed of benzene, fluorene, carbazole, and thiophene units, as a p-bridge have been synthesized and characterized by optical, electrochemical, and theoretical investigations. The trends in the absorption and electrochemical properties of these dyes are in accordance with the electron-donating ability of the conjugating segment. Consequently, the dyes that contained a 2,7-carbazole unit in the p-spacer exhibited red-shifted absorption and lower oxidation potentials than their corresponding fluorene- and phenylene-bridged dyes. However, the enhanced power-conversion efficiency that was exhibited by the fluorene-bridged dyes in the DSSCs was attributed to their broader and intense absorption. Despite the longer-wavelength absorption and reasonable optical density, carbazole-bridged dyes exhibited lower power-conversion efficiencies, which were ascribed to the poor alignment of the LUMO level in these dyes, thereby leading to the inhibition of electron injection into the TiO2 conduction band.